The Total Fertility Rate (cont.)
Cohort Fertility
Cohort fertility is the fertility (the number of live births) produced by a particular group of women throughout their reproductive life i.e. a real cohort. It is common to work with birth cohorts although other types of cohort e.g. marriage cohorts are possible. Once they reach age 50 their cohort fertility can be assessed. If the average number of live births is 4.2 then this can be expressed as a Total Cohort Fertility Rate of 4.2. It is more often labelled as the Completed Family Size so that it is not confused with the period TFR.
Cohort fertility is more stable than period fertility but has a disadvantage that it relates to an earlier period of time and is not current. A group of women born between 1945 and 1949 will begin their reproductive years in 1960-64 and finish them 1995-99. If in that period they had 4.2 live births on average then that is their Completed Family Size – but where does this figure relate to in time? If we had to plot it on a graph to compare against a time series of period TFRs where would we plot it?
The answer is that we would plot it against the year that represents the mean of the fertility distribution or the mean age of childbearing. These are slightly different measures but usually very close at an age of about 28 years – we would therefore plot the 4.3 figure as being “centred” in, say, 1975 – so not at all current.
A series of cohort fertility measures plotted like this usually shows much less variability than a series of period TFRs and therefore shows a truer picture of fertility quantum.
The following pages give an idea of how period and cohort measures relate to each other. They introduce a useful aid called the Lexis Diagram. An actual example of how period rates can be viewed in a cohort way will be presented later.